The present invention relates generally to flow measurement in chemical analysis systems and, more particularly, to a chemical preconcentrator with an integral thermal flow sensor for flow measurement in a microanalytical system.
Portable, handheld microanalytical systems, which have been termed xe2x80x9cchemical laboratories on a chip,xe2x80x9d are being developed to enable the rapid and sensitive detection of particular chemicals, including pollutants, high explosives, and chemical and biological warfare agents. These microanalytical systems should provide a high chemical selectivity to discriminate against potential background interferents and the ability to perform the chemical analysis on a short time scale with high selectivity. In addition, low electrical power consumption is needed for prolonged field use.
Current gas-phase microanalytical systems are based on gas chromatography (GC). Such microanalytical systems can also include a chemical preconcentrator. The chemical preconcentrator serves the important function of concentrating and purifying a chemical sample on a sorptive material at the inlet of the microanalytical system. The chemical preconcentrator can deliver an extremely sharp sample plug ( less than 200 msec full-width at half maximum) to the downstream gas chromatograph by taking advantage of the rapid, efficient heating of the sorped chemical sample with a low-heat capacity, low-loss microhotplate.
Knowledge of the fluid flow rate in a microanalytical system is often required for an accurate interpretation of the system""s response to a chemical species. For example, the retention time for chemical species depends directly on the gas flow rate in the microfabricated GC column. In addition, knowledge of the flow rate during sample collection is important to determine the total volume of gas that was sampled. This is information that can provide accurate determination of the concentration of analyte in the sampled gas. Finally, the height per theoretical plate obtained for microfabricated GC columns varies with gas flow rate. Therefore, the proper interpretation of results obtained with a microanalytical system containing a microfabricated GC depends critically on the knowledge of the gas flow rate during the time in which the chromatogram was taken.
Microfabricated thermal flow sensors have been developed that rely on the measurement of forced-convective heat loss from a heated element placed in the flow stream. See, e.g., M. Elwenspoek and R. Wiegerink, Mechanical Microsensors, Springer-Verlag, Berlin (2001). In a thermal-anemometer-type flow sensor, an element having a temperature-dependent resistivity is heated in a flow field. The heated element is typically a free-standing, thermally isolated structure that can function as a thermal flow sensor. Any flow across the heated element will increase the heat loss from the element due to forced convection. Thus, the larger the flow velocity is, the more heat will be lost from the heated element. The thermal flow sensor can be operated in a constant voltage or power mode wherein the temperature of the heated element is measured as the flow over the sensor is varied. With increased gas flow, the increased heat loss will lead to a reduction in the temperature of the element. The resulting decrease in the element""s resistance can be measured with electrical instrumentation and related back to the flow velocity over the element. Alternatively, the thermal flow sensor can be operated in a constant temperature mode wherein the power needed to keep the temperature of the element constant in a fluid field is measured.
The thermal-anemometer-type flow sensor is ideally suited for microelectromechanical systems (MEMS). Such MEMS-based thermal flow sensors have the advantages of high precision, small size, low power consumption, high sensitivity, low response time, and batch production. In particular, such MEMS-based thermal flow sensors can be easily integrated with microanalytical systems because the same microfabrication technology can be used to construct all components of the microanalytical system, including the thermal flow sensor.
The chemical preconcentrator with integral thermal flow sensor of the present invention achieves the goals of accurately measuring the fluid flow rate in a microanalytical system while retaining system portability and functionality. The thermal flow sensor can be fabricated with the same MEMS technology as the rest of the microanalytical system, enabling easy integration with other microanalytical system components and the necessary control and sense electronics. Moreover, the same electronics that are used for desorption control of the chemical preconcentrator can be used to sense flow over it. Because this is a low-heat capacity, low-loss, miniature thermal flow sensor, it is fast and efficient enough to be used in battery-powered, portable systems.
The present invention comprises a chemical preconcentrator with integral thermal flow sensor. The chemical preconcentrator has a substrate having a suspended membrane formed thereon and at least one resistive heating element, comprising an electrically-conductive material whose resistance varies with temperature, disposed on a surface of the suspended membrane, and wherein the suspended membrane is exposed to a flow channel of the chemical preconcentrator. The thermal flow sensor can have a control circuit for heating the resistive heating element to a set temperature and measuring the power required to maintain the resistive heating element at the set temperature when a fluid flows in the flow channel. Alternatively, the control circuit can apply a set voltage across the resistive heating element and measure the resistance of the resistive heating element when a fluid flows in the flow channel.